mftransform: Fix deadlock when MFT requests processing output twice
This sequence of event/data flow might happen
1) Initially we have one pending output event
1-1) Then, process the pending output data
2) No pending input event, then we should wait new pending input event
2-1) Wakeup by new pending event (but it's pending output event)
In above case, MFT will not report new pending input event
if pending output is not processed.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1325>